Figure 1. WINEXRAY's representative berry profile of Cabernet Sauvignon from the north coast of the United States. (The Future of Winemaking: Honoring the Vision of Professor Roger Boulton, 2022).

EXTRACTABLE ANTHOCYANINS (mg/berry)

Broadly speaking, great wines have grown where moderate stress leads to higher concentrations of color, tannin, and flavor. The best vineyards have been where phenolic ripeness and flavor ripeness coincide. For millennia, the earth’s climate has been stable and wine traditions have slowly formed to best fit that mold. Today, with increasing weather volatility, winemakers need to account for mitigating the risk of extreme environmental stress to preserve their site's equilibrium. Following traditions is no longer a given, it is an ideal. Understanding the behavior of grape and wine phenolics in changing climate conditions is fundamental for making informed decisions. As always, we begin in the vineyard to understand the quality inherent to site.

BERRY ANALYSIS

The status quo for quantifying berry ripeness is by Brix, pH, and TA. These fundamental metrics are valuable information for production, but they are also limited in their scope. To gain a much deeper understanding we look towards three other parameters: berry sugar loading, fresh berry weight, and extractable anthocyanins. The rise and decline of these three values all tend to coincide together, a very valuable piece of information for winegrowers seeking to track berry ripeness.

BERRY SUGAR LOADING (mg/berry)

The accumulation of anthocyanins and sugars in grapes coincide together because all anthocyanins contain glucose in their chemical structure. Sugar accumulation signals anthocyanidin accumulation and enzymes synthesize them together (Das et al., 2012; Ma et al., 2021; Walker et al., 2021). Berry sugar loading is a way to track sugar accumulation irrespective of dehydration. It is simply the average dry weight of sugar per berry. The traditional measurement of °Brix measures the dissolved solids relative to berry water, thus °Brix increase with berry dehydration. A good example to distinguish the two measurements is during a heat wave. Under extreme stress, vines use grapes as sinks and metabolize their sugars. While the °Brix increase due to dehydration, berry sugar loading decreases due to vine stress. Winegrowers can gain much more information about the metabolism, resiliency, and recovery of their vineyard by tracking berry sugar loading as a metric of vine metabolism than simply tracking °Brix.

FRESH BERRY WEIGHT (mg/berry)

Fresh berry weight is simply the average individual berry weight. It is a powerful metric for evaluating growth, dehydration, yield, and consistency over time. In particular, fresh berry weight can be used as a reference point for irrigation depending on what style of wine a producer hopes to achieve.  

EXTRACTABLE ANTHOCYANINS (mg/L)

Figure 1. Anthocyanins prevalent in Vitis vinifera colored by their respective hues.

“Phenolic compounds are secondary metabolites. They are not involved in growth and energy metabolism and are usually generated in response to environmental stress (e.g., predation, attack by microorganisms, UV light levels, etc.)” (Harnly et al., 2007). By measuring the accumulation of extractable anthocyanins in grapes, we can understand the environmental conditions that favor their formation. For instance, moderate environmental stress may favor accumulation (<29°C/85°F), but extreme weather events like heat waves (>43°C/110°F) significantly degrade anthocyanins and tannins alike (WINEXRAY). We value extractable anthocyanins because they play a particularly important role in crafting red wine style by modulating astringency, incorporating flavor, and stabilizing color. Their quality and quantity in grapes are determined by a matrix of growing variables, most notably light, temperature, cultivar, and water. 

Synthesis

Extractable anthocyanins are a profound metric for understanding color potential, grape ripening, and extraction efficiency (when compared to total anthocyanins in wine). The berry is its own biochemical reactor. The soil and vine supply water, sugar, amino acids, minerals, and micronutrients, but the berry is solely responsible for synthesizing all secondary metabolites, including phenolics and flavors (Kennedy, 2002). Anthocyanins consist primarily of free anthocyanin monomers as well as some bound anthocyanin polymers (generally <50 ppm) which synthesize from the start of veraison to the end of ripening. This is key because sugar accumulation signals anthocyanin synthesis from chemical precursors. Enzymes bind anthocyanidins and glucose together to form pigmented anthocyanins.

Of the 23 anthocyanins found in vascular plants, V. vinifera produces only 6 anthocyanins: malvidin-3-O-glucoside; petunidin-3-O-glucoside; delphinidin-3-O-glucoside; peonidin-3-O-glucoside; cyanidin-3-O-glucoside; and pelargonidin-3-O-glucoside. All anthocyanins have a glucose molecule attached to them, hence the “glucoside” suffix. Figure 1 shows the chemical structure of each of these compounds, and it is very interesting to see their structural similarity. All monomeric anthocyanins from V. vinifera are monoglucosidic, meaning they have one glucose. Once extractable anthocyanins reach their peak, they rapidly begin to fall. While certain viticultural practices such as extended hang time have proven to build bound anthocyanins in grape skins, the amount is generally nominal compared to what we can form in the winery.

Anthocyanin Stability

It’s important to note that not all 6 anthocyanins are made equal. Malvidin is the most abundant, the most purple, and the most stable anthocyanin found in most V. vinifera cultivars (Mori et al., 2007). An example of this can be seen in Figure 2 below. This study performed on 11-year-old potted Cabernet Sauvignon showed a 100% increase in anthocyanins accumulation when grapes were grown at 25°C (77°F) versus 35°C (95°F) during the daytime (14 hours). Both trials were reduced to 20°C (68°F) during the nighttime (10 hours). All anthocyanins except Malvidin-3-O-glucoside were drastically reduced.

Figure 2. Anthocyanin accumulation in 11-year-old potted Cabernet Sauvignon (Mori et al., 2007).

Conclusion

Grape extractable anthocyanins are finite and important stylistic drivers for red winemaking. By measuring extractable anthocyanins, we enhance our perspective of grape evolution in the vineyard. At present, grape phenolic analysis is still a slow and laborious process. This is not to detract from the value of information that the analysis provides; if anything, we need it more than ever. Extractable anthocyanins are a powerful indicator of vine metabolism and color potential in grapes. Given their instability, we focus on their concentration rather than their tannin counterparts. Pairing extractable anthocyanins with berry sugar loading and fresh berry weight gives winemakers a deep insight into vine health and the character of the vintage. In short, it is an additional vantage point from which we can observe nature. We characterize the end of ripening as the end of metabolite synthesis, something that all winemakers can track via berry sugar loading and/or extractable anthocyanins measurement.

References 

Colantuoni, G., McLeod, S. WINEXRAY LLC. https://www.winexray.com/

Das, P. K., Shin, D. H., Choi, S.-B., & Park, Y.-I. (2012). Sugar-hormone cross-talk in anthocyanin biosynthesis. Molecules and Cells, 34(6), 501–507. https://doi.org/10.1007/s10059-012-0151-x

Harnly, J. M., Bhagwat, S., & Lin, L.-Z. (2007). Profiling methods for the determination of phenolic compounds in foods and dietary supplements. Analytical and Bioanalytical Chemistry, 389(1), 47–61. https://doi.org/10.1007/s00216-007-1424-7

Kennedy, J. (2002). Understanding grape berry development. Practical Winery and Vineyard, 24.

Ma, Y., Ma, X., Gao, X., Wu, W., & Zhou, B. (2021). Light Induced Regulation Pathway of Anthocyanin Biosynthesis in Plants. International Journal of Molecular Sciences, 22(20), 11116. https://doi.org/10.3390/ijms222011116

Mori, K., Goto-Yamamoto, N., Kitayama, M., & Hashizume, K. (2007). Loss of anthocyanins in red-wine grape under high temperature. Journal of Experimental Botany, 58(8), 1935–1945. https://doi.org/10.1093/jxb/erm055

Walker, R. P., Bonghi, C., Varotto, S., Battistelli, A., Burbidge, C. A., Castellarin, S. D., Chen, Z.-H., Darriet, P., Moscatello, S., Rienth, M., Sweetman, C., & Famiani, F. (2021). Sucrose Metabolism and Transport in Grapevines, with Emphasis on Berries and Leaves, and Insights Gained from a Cross-Species Comparison. International Journal of Molecular Sciences, 22(15), 7794. https://doi.org/10.3390/ijms22157794